This application claims the priority of Japanese Patent Application No. 2000-135368 filed on May 9, 2000, which is incorporated herein by reference.
The present invention relates to a vehicular automatic transmission (including a continuously variable transmission) which is constructed to perform an automatic control for establishing speed change ratios in response to changes in the driving condition while the driver manipulates the shift lever to select drive ranges.
A vehicular automatic transmission comprises a speed change mechanism which includes a plurality of mechanical power transmission paths and is connected to an engine through, for example, a torque converter. Selection of or switching to an appropriate power transmission path, i.e., an appropriate speed change ratio, is automatically performed on the basis of, for example, the accelerator opening and the vehicle speed. Generally, a vehicle with an automatic transmission is equipped with a shift lever, which is operated by a driver. When the driver manipulates the shift lever, a range of speed change ratios or a drive range (for example, a reverse drive range, a neutral range, a forward drive range) is selected in response to the manipulation of the shift lever, and the speed change ratio is automatically adjusted or controlled within the selected drive range (usually in a forward drive range).
When a vehicle equipped with such an automatic transmission is stopped while the transmission is set in the forward drive range, a so-called creeping phenomenon occurs because the driving force from the engine which is idling is transmitted to the transmission through the torque converter and then to the wheels. This creeping phenomenon is beneficial for smooth starting under a certain condition, for example, when the vehicle stops and then starts on an upward slope, but not welcome when the vehicle should be kept stationary. At present, to keep the vehicle stationary, the brake must be operated against the creeping force, which is generated by the engine, so this brake operation reduces the mileage allowance or the fuel efficiency of the vehicle.
To solve this problem, a method to improve the fuel efficiency of such an automatic transmission is proposed. In this method, while the vehicle is kept stationary with the brake pedal being stepped or pressed for brake operation and the accelerator being substantially closed, the transmission is set into a neutral condition although the selection of the drive range is kept unchanged (for example, Japanese Laid-Open Patent Publication Nos. H11-193866, H11-230329, etc.).
By the way, it is a difficult task to make the vehicle start quickly and smoothly without any delay from the stationary condition in which the transmission is set in the above mentioned neutral condition in the forward drive range when the brake pedal is released for a creeping and then the accelerator pedal is stepped. Especially, while the vehicle is stationary on an upward slope and when the brake is released, if there is a delay in the response of the transmission to switch from the neutral condition to a drive condition that transmits the driving force to the wheels, then the vehicle moves backward a little depending on the inclination of the road before it starts traveling forward. If such a backward movement occurs, then the driver may feel that something is wrong with the shift control system of the transmission.
It is an object of the present invention to provide a control system for a vehicular automatic transmission, which system enables the vehicle to creep or start quickly and smoothly from a stationary condition in which the transmission is set in a neutral condition in a forward drive range when the brake pedal is released.
Particularly, it is an object of the present invention to provide a control system for a vehicular automatic transmission, which system performs a quick and flawless shift control to establish a forward drive condition from a neutral condition in which the vehicle has been kept stationary on an upward slope, with little moving back of the vehicle when the brake is released.
To realize the above objectives, the present invention provides a control system (for example, the control valve CV and the electronic control unit ECU described as a preferred embodiment in the following section) for a vehicular automatic transmission (for example, the automatic transmission TM described in the following section). The control system according to the present invention performs an automatic control for selecting and establishing a speed ratio (speed change gear ratio) in response to the condition of the vehicle set in a drive range, and the control system creates a neutral condition if the vehicle is stationary with the brake of the vehicle being actuated and the accelerator for the engine being turned off (i.e., the accelerator pedal is released to turn off the accelerator, but the engine is kept idling) while the transmission is set in the drive range. This control system is equipped with an engine output control device (the engine output control device 50 described in the following section) that adjusts the engine output being input into the transmission when the transmission leaves the neutral condition and starts to establish a predetermined speed ratio in the drive range.
It is preferable that the control system include an inclination detector (the inclination sensor 37 described in the following section) which detects the inclination of the vehicle in the direction of the vehicle""s traveling. With this arrangement, the engine output control device adjusts the engine output in correspondence to the inclination detected by the inclination detector. In this case, if the inclination in the vehicle""s traveling direction detected by the inclination detector indicates that the road surface is upwardly sloped, then the engine output control device preferably increases the engine output. On the other hand, if the inclination in the vehicle""s traveling direction detected by the inclination detector indicates that the road surface is downwardly sloped, then the engine output control device preferably decreases the engine output.
The control system according to the present invention can control quickly and smoothly the operation of the transmission from a neutral condition where the vehicle is stationary on an upwardly sloped road to a driving condition where the brake pedal is released to make the vehicle start or creep by adjusting appropriately the power from the engine which is input into the transmission by the engine output control device. For example, in a case where the vehicle has stopped on an ascending road, the control system can increase the engine output by the engine output control device to achieve a relatively large creeping force. In this way, the vehicle is prevented from receding, which may otherwise happen depending on the magnitude of the inclination, or the amount of the receding can be greatly reduced. As a result, the starting and the creeping of the vehicle is controlled without any clumsiness and with comfort.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.